1. Field of the Invention
The present invention relates to a method of recovering carbon dioxide gas present in combustion exhaust gas. More specifically, it relates to carbon dioxide recovery in a power plant provided with equipment therefor and which performs steam power generation while carbon dioxide is recovered from the combustion exhaust gas from the boiler, and relates also to an efficient power generating method therefor. Also, it relates to a method for absorbing and recovering carbon dioxide contained in combustion exhaust gas, using an aqueous solution of monoethanolamine as an absorbing solution. Further, it relates to a method for recovering carbon dioxide from combustion exhaust gas which comprises cooling combustion exhaust gas to a given temperature range by wet cooling or other method, absorbing carbon dioxide contained in the gas using an aqueous solution of monoethanolamine as an absorbing solution, and then recovering the carbon dioxide from the aqueous monoethanolamine solution.
2. Description of the Related Art
In recent years the greenhouse effect of carbon dioxide has arrested attention as a factor responsible for the global warming phenomenon, and a measure against it is urgently sought in worldwide efforts to protect the environments on the earth. The source of carbon dioxide is omnipresent in every field of human activities that involve the combustion of fossil fuels, and the trend is heading toward stricter emission control of carbon dioxide. In view of these, energetic studies are under way on the recovery of carbon dioxide from combustion exhaust gases, especially from those emitted from power-generating installations such as steam power plants that burn large volumes of fossil fuels, and on the storage of the recovered carbon dioxide without discharging it to the atmosphere.
As a way of recovering carbon dioxide from combustion exhaust gas containing carbon dioxide, a system illustrated in FIG. 6 has already been proposed. FIG. 6 shows only major components and omits auxiliaries.
FIG. 6 shows a turbine-driven generator 2 into which steam produced by a boiler 1 is conducted via line 3 for power generation. The combustion exhaust gas emitted by the boiler 1 is led through line 4 to a cooler 5, where it is cooled by contact with cooling water, and the cooled gas is transferred through line 6 to an absorber 7. The absorber 7 is supplied, from its top, with an aqueous solution of monoethanolamine at a concentration of about 20 to 30 percent by weight via line 8. The aqueous monoethanolamine solution falls in countercurrent contact with the combustion exhaust gas, takes up carbon dioxide from the gas, and, as an aqueous monoethanolamine solution containing the absorbed carbon dioxide, flows out at the bottom of the absorber column 7 and is led through line 9 to an aqueous monoethanolamine regenerator 10. The combustion exhaust gas from which carbon dioxide has been removed by absorption is released from the top of the absorber 7 to the atmosphere through line 11.
Inside the aqueous monoethanolamine regenerator 10, heating with steam from a reboiler 13 regenerates the aqueous monoethanolamine solution that has absorbed carbon dioxide, and the regenerated solution is returned to the absorber 7 via line 8. Carbon dioxide is conducted through line 14 to a recovery step. Where necessary, a heat exchanger may be installed to effect heat exchange between the lines 8 and 9. For a heat supply to the reboiler 13, either steam produced by the boiler 1 or low pressure-side steam extracted from the turbine-driven generator 2 is conducted through line 12 to the reboiler.
In the above-described system, while thermal generation is in progress, carbon dioxide is recovered by absorption from the combustion exhaust gas, and the amount of steam consumed by the reboiler 13 accounts for as much as about 20 percent of the total steam production by the boiler 1. On the other hand, the demand for electricity varies widely within a day. The demand is high especially in the daytime, from about ten o'clock in the morning to about five in the afternoon. Thus, it is important to boost power supply sufficiently for this peak period. In reality, as noted above, about 20 percent of the steam supply for power generation must be set aside for the regeneration of the aqueous monoethanolamine solution, with a corresponding reduction of power generated. A solution to this problem has been sought.
Also, prior to the current public interest in the recovery of carbon dioxide gas present in combustion exhaust gases, in order to absorb and remove carbon dioxide from combustible gases, such as natural gas, ammonia gas, and hydrogen gas, monoethanolamine had been used and it is still in use for such purposes. Monoethanolamine is usually used as a low-concentration aqueous solution containing 40 percent or less by weight and is not flammable itself. Flammable is the stock solution used to replenish the monoethanolamine consumed during the recovery of carbon dioxide or to adjust the concentration of the absorbing solution. The stock solution is customarily stored in the form of either a high-concentration aqueous solution or undiluted, 100% monoethanolamine in a tank or the like within the recovery equipment. To minimize the storage tank volume, storage is preferably done in its undiluted form. However, the 100% monoethanolamine has a solidifying temperature of 10.5.degree. C., and in order to avoid the solidification in cold weather it is sometimes replaced by a flammable aqueous solution at as high a concentration as about 85 percent by weight (nonfreezing grade). Inasmuch as the equipment for recovering carbon dioxide from such a combustible gas as referred to above is designed primarily to handle the combustible gas, all the motors, measuring instruments, electric facilities, etc. are made explosion-proof. Fire-fighting arrangements, of course, are also provided. Thus, it has been unnecessary to consider extra protection against explosion hazards in storing the flammable stock solution of monoethanolamine at such a high concentration.
However, as we described above, as the absorption of carbon dioxide gas from combustion exhaust gas is now gathering more attention, the following problems have been noted. That is, with an equipment for absorbing carbon dioxide out of combustion exhaust gas, the situation is utterly different from that for a conventional apparatus for recovering carbon dioxide from combustible gases. The gas which has to be dealt with is nonflammable by nature. Practically the only flammable matter that requires explosion-proof arrangements and extinguishing facilities is the stock solution of monoethanolamine for use in replenishing the monoethanolamine consumed for the recovery of carbon dioxide or for use in adjusting the concentration of the solution. However, because the flammable stock solution is stored and used within, the carbon dioxide recovery equipment must have many components guarded against explosion, with the installation of fire extinguishers, as is the case with the above-mentioned equipment for carbon dioxide recovery from combustible gas. Needless to say, the use of explosion-proof motors, measuring instruments, and electric facilities, plus extinguishers, is much costlier than the adoption of a system without explosion-proofing.
Furthermore, when carbon dioxide gas is absorbed from combustion exhaust gas using a monoethanolamine solution, the following problems have been noted with respect to the temperature of the exhaust gas.
FIG. 5 shows the saturation curve under the partial pressure of carbon dioxide accounting for 8 percent by volume of the atmosphere, when an aqueous solution of monoethanolamine at a concentration of about 30 percent by weight is used for absorbing carbon dioxide contained in combustion exhaust gas. The abscissa of FIG. 5 is the temperature (.degree.C.) and the ordinate is the number of moles of carbon dioxide absorbed per unit mole of monoethanolamine. As is clear from FIG. 5, the lower the temperature of carbon dioxide that comes in contact with the solution, the larger the amount of carbon dioxide absorbed by the aqueous monoethanolamine solution becomes. Another reason for which the lower gas temperature is preferred when the gas contacts with the aqueous monoethanolamine solution is that the absorption of carbon dioxide by the monoethanolamine solution entails heat generation. It has therefore been believed necessary to cool the gas downright, e.g., to the range of about 30.degree. C. to 50.degree. C. Thus the conventional equipment for the recovery of carbon dioxide from combustion exhaust gas uses an expensive gas cooling installations.
The operation of the recovery equipment too is quite costly because a heat exchanger or the like must be used to cool the cooling water for such gas cooling installations. In particular, combustion exhaust gas from a boiler or other furnace that burns natural gas poses problems that do not arise from exhaust gases from the combustion of coal or heavy oil. For example, the gas temperature will not come down readily upon mere wetting which is done by bringing the gas into contact with cooling water; the gas must cooled by contact with water which is particularly cooled using a heat exchanger.